Remove using directives from the generated C# code and fully qualify the use of all .NET framework types in order to minimize potential name collisions from input files defining types, namespace, etc with the same name as .NET framework members.

Examples/test-suite/special_variable_macros.i

@@ -188,7 +188,7 @@ namespace Space {
 
 #if defined(SWIGCSHARP)
 %typemap(cscode) Space::RenameMe %{
-  public static NewName factory(String s) {
+  public static NewName factory(System.String s) {
   //below should expand to:
   //return new NewName( new Name(s) );
     return new $typemap(cstype, Space::RenameMe)( new $typemap(cstype, Name)(s) );